1. Field of the Invention
The present invention relates to a distributed reflector and a semiconductor laser with such a distributed reflector useful in optical transmission field, and more particularly to a wavelength-tunable semiconductor laser with a distributed reflector which is suitable as a light source for transmission, a tunable light source for synchronous rectification, and a light source for photometry in optical wavelength (frequency) multiplexing communication systems.
2. Description of the Prior Art
As a wavelength-tunable light source, distributed reflector type semiconductor lasers have been studied widely. FIG. 1A is a schematic cross-sectional view showing a basic structure of a first conventional example of a wavelength-tunable distributed reflector type semiconductor laser. (e.g., Y. Tohmori et al., Electronics Letters, Vol. 19, p. 656 (1983)). In FIG. 1A, 2 is an active waveguide layer, 3 an inactive waveguide layer, 20 a diffractive grating, 101 an active region, 102 a front side distributed reflection region, 103 a rear side distributed reflection region, and 104 a phase adjustment region. In such a type of laser, the distributed reflectors on the both front and rear sides have identical peak wavelengths and can lase stably at a low threshold value when their respective reflectivities are high.
However, in the first conventional example, the distributed reflector regions 102 and 103 each may be deemed to have a uniform pitch .LAMBDA., and a uniform equivalent refractive index, n.sub.eq, as illustrated in FIG. 1B. Hence, Bragg wavelength, .lambda..sub.B, at which a high reflectivity is obtained will be observed as a single line as illustrated in FIG. 1C. Tunable range of a lasing wavelength is an amount of variation .DELTA..lambda. (=2.LAMBDA..multidot..DELTA.n.sub.eq) from the Bragg wavelength, .lambda..sub.B, and depends on the amount of variation .DELTA.n.sub.eq of the equivalent refractive index, n.sub.eq, caused by current injected into the distributed reflector region. Incidentally, a maximum relative equivalent refractive index variation, .DELTA.n.sub.eq /n.sub.eq, is 1% or less, and various workers on experiments on wavelength tunability with the above-mentioned conventional distributed reflector type semiconductor lasers (cf., e.g., Y. Kotaki, et al., Electronics Letters, Vol. 23, p. 327 (1987)) remained to report wavelength tunable ranges on the order of 10 nm, which is insufficient for providing a light source for optical wavelength multiplexing communication systems.
Recently, a distributed reflector as shown in FIG. 2A has been proposed which enables wavelength tuning within the range on the order of above 10 nm, which is referred to herein as reflector according to a second conventional example. In FIG. 2A, broken line indicates portions whose depiction has been omitted. In this case, the waveguide has a diffractive grating of a pitch of .LAMBDA., formed partially and periodically, thereby giving a plurality of reflection peaks in the vicinity of Bragg wavelength, .lambda..sub.B, which depends on the pitch, .LAMBDA., as illustrated in FIG. 2B. In a laser structure, one of the reflection peaks is selected electrically (i.e., rough adjustment of wavelength), and wavelength tuning is performed in the vicinity of the selected reflection peak (i.e., fine adjustment of wavelength) (cf. U.S. Pat. No. 4,896,325 to L. A., Coldren et al.; and V. Jayaraman et al., LEOS, "91, paper SDL15.5, 1991). There is a report on wavelength tuning which says only on an experimental basis, rough adjustment on the order of 50 nm was made (V. Jayaraman et al., IEEE, 13th International Conference on Semiconductor Lasers, Post Deadline Papers PD-11).
In the second conventional reflector, as will be understood from FIG. 2B, reflectivities decrease considerably as the wavelengths depart from Bragg wavelength, .lambda..sub.B. There has been no report on continuous operation of a wavelength-tunable semiconductor laser having the above-mentioned reflector according to the second conventional example while only pulse operation was made to confirm its action.
With view to complying to expected rapid and great increase in the amount of communication and information in future, various investigations have been made on optical wavelength (frequency) multiplexing communication systems and there is a keen desire on the development of a light source with a wavelength tunability over a wide range as a light source for transmission, and synchronous rectification. Also, it is desired to realize a wavelength-tunable light source which can cover a wide band useful in the field of photometry.